This invention relates to trusses for roofs, specifically to such roof trusses that provide storage space or living space.
There are a large variety of roof truss designs. They are extensively used in residential buildings due to their relative low cost, prefabrication, and rapid installation. Trusses are planar and joined arrangements of straight structural elements, chords and webs. In residential buildings the chords or webs are usually pieces of dimension lumber, 2-by-4s or 2-by-6s. Chords are the elements on the outside edge of the truss. Webs are the elements on the interior of the truss. The most common exterior shape of a roof truss is a symmetrical or isosceles triangle. A horizontal bottom chord is on the bottom of the triangular shape. Each end of the bottom chord terminates at a heel and is joined to an inclined top chord. The top chords extend from the heel upward and toward each other and are joined at an apex. Trusses are generally supported at their heels. There are a large variety of trusses with other than symmetrical triangular exterior shapes, such as Scissors, Mono, Parallel Chord, Gambrel, Hip, etc.
Most trusses have their chords and webs arranged in triangular mesh patterns with the triangle vertices coincident with at least one other vertex (except at the heels). A triangle vertex does not terminate between the vertexes of another triangle. This is a very structurally efficient design. Loads on a triangle vertex (chord and web junctures) are resisted by compression or tension of the chords or webs forming the triangle. Loads between vertices are resisted by bending of the chord or web. For the same load this requires chords or webs with much larger cross sections. The arrangement of triangles with coincident vertexes causes loads to be transferred via compression or tension of the chords or webs. Chords support live and dead loads such as wind, snow, roof or ceiling membrane weight etc. The loads on the chords are transferred to the junctures between chords and webs.
The xe2x80x9cWood Engineering and Construction Handbook, 3rd Ed.xe2x80x9d by Faherty and Williamson shows 34 typical trusses (on pages 6.7 and 6.8). All but three of these truss designs have a triangular mesh pattern of their webs and chords. One of the three is a Gable End which is continuously supported on its bottom cord and therefore is not a load carrying roof truss. The second of the three is an Attic truss, its"" limitations are discussed below. The third of the three is called a FlorTrus(trademark). The FlorTrus is a flat truss (parallel top and bottom chords) with a small central rectangular opening. The opening is to allow passage of heating and cooling air ducts. The bending loads in the top and bottom chords around the opening are tolerable because of the small span of the opening. This design is not suitable for a storage space.
xe2x80x9cAppendix G of ANSI/TPI 1-1995 National Design Standard for Metal Plate Connected Wood Truss Constructionxe2x80x9d shows 42 types of trusses. All but five of these truss designs have a triangular mesh pattern of their webs and chords. One of the five is a Gable End, as described above, which is continuously supported on its bottom cord and therefore is not a load carrying roof truss. The second of the five is a Room-in-Attic truss, which is the same as the Attic truss discussed above. The limitations of the Room-in-Attic or Attic truss are discussed below. The third of the five is called a Warren truss. This truss is identical to the FlorTrus described above. The Warren truss is a flat truss (parallel top and bottom chords) with a small central rectangular opening. The opening is to allow passage of heating and cooling air ducts. The bending loads in the top and bottom chords around the opening are tolerable because of the small span of the opening. This design is not suitable for a storage space. The fourth of the five is a Fan or Modified Warren truss. This truss is a Warren truss with additional vertical webs, its"" limitations are the same as the Warren truss. The fifth of the five is a xe2x80x9cDouble Cantilever with Parapetsxe2x80x9d. The design is a flat truss with sloped cantilevered ends that form parapets. This truss has a triangular mesh pattern of webs and chords except above the support points. At this location a triangle vertex terminates on another triangles"" side rather than its"" vertex. This design is not suitable for storage space.
Toothed metal connector plates or truss plates such as shown in U.S. Pat. Nos. 2,844,852 to West and 3,473,362 to Black et al are commonly used to connect wood truss elements. The plates are place on each side of a joint with the teeth facing the wood and then pressed in. This forms a strong and very compact joint. Prior to truss plates plywood gussets were used. Plywood gussets were placed on both sides of the joint then glued and nailed to the chords and webs. Pins can form joints such as shown in U.S. Pat. No. 5,722,210 to Baker et al.
U.S. Pat. No. 732,787 to Scheidler shows the frame for a barn with a large central opening with vertical lower sides. The roof is gambrel shaped, the upper portion has less pitch or slope than the lower portion. The barn sides, the lower and steeper roof portion, and the lower half of the upper roof portion are supported by a triangular mesh pattern of webs and chords. The top chords of the upper roof cantilever out from the triangular mesh and are joined at the apex. There is no bottom chord to tie the sides together. This design could not be used as a roof truss.
The most common truss used in residential construction is the fink or W truss shown in FIG. 11. The W truss has a large triangular central opening 36 with a horizontal bottom chord 26 and a pair of apex webs 30 forming the sides. Decking material can be placed over the horizontal bottom chord of the triangular patterns of several trusses forming a storage space. Other truss configurations have horizontal bottom elements or chords and can be used to form a storage space. However, a W type of truss has the largest possible triangular space with a horizontal bottom element for the same width and height of truss.
Low overhead usually limits this space to storage versus a living space. The sloping sides of the triangle limit usable storage space. Items with any height cannot be pushed to the comers of the triangle. The center of the triangle is normally kept clear to provide a path or passageway to the stored items. The path limits the usable storage space. The steep slope of these sides makes the path or passageway feel smaller which inhibits usage. People feel confined and therefore are less likely to use the space.
An Attic truss shown in FIG. 12 provides storage or living space. The center opening 36 is defined by a bottom chord 26, vertical webs 28, portions of the top chords 22, and a cross web 42. The vertical lower sides, provide by the vertical webs, greatly add to the usable space. FIG. 13 shows another version of the Attic truss, sometimes called Room-in-Attic truss. The center opening 36 is defined by a horizontal bottom chord 26, a cross chord 56, and vertical webs 28. The chords and webs for either type of attic truss do not have the required triangular mesh pattern for greatest structural efficiency. The chords must carry significant joint loads via bending and therefore must be of greater cross section. Larger spans of the either type of Attic truss require much greater cross section of the chords, therefore this design is usually only seen in shorter span designs. To provide adequate headroom both types of Attic trusses must have steeply inclined top chords.
Truss elements, chords and webs, are arranged to provide a central opening with a horizontal bottom and vertical lower sides. This allows more usable storage space. The central opening chords and webs are connected to the top chords with webs and chords forming triangular mesh patterns such that loads on the chord and web junctures are carried in tension or compression. Therefore structural efficiency and cost is similar to a conventional truss.
The storage roof truss provides a large usable storage space within the trusses. Flooring or decking placed across the horizontal bottom chord of multiple trusses provides storage space. The vertical lower sides of the storage truss""s central opening provide usable space over the entire decked area. A prior art Attic truss provides storage or living space. The Attic truss requires top chords with a high pitch or large incline and larger cross sections of top and bottom chords. Loads placed on the attic truss junctures are not carried principally in tension or compression of the chords but are carried as moment or bending loads therefore chord cross sections must be larger. The triangular mesh pattern of storage roof truss chords and webs between the central opening and the top chords cause loads placed on the element junctures to be carried in tension or compression. This is the same structural efficiency that conventional trusses have. Therefore truss chords with small cross sections can be used allowing low cost.